1. Field of the Invention
The invention relates to an exhaust purification apparatus of an internal combustion engine and a control method thereof.
2. Description of the Related Art
In a known 6-cylinder internal combustion engine (see, for example, Japanese Patent Publication Application No. JP-A-2004-68690), the cylinders are divided into a first cylinder group made up of three cylinders and a second cylinder group made up of the other three cylinders, and the cylinders of the first cylinder group are connected to a common first exhaust passageway, and the cylinders of the second cylinder group are connected to a common second exhaust passageway. Each of the first exhaust passageway and the second exhaust passageway is provided with an air-fuel ratio sensor and a three-way catalyst. Downstream of these three-way catalysts, the first exhaust passageway and the second exhaust passageway are connected to a common NOx storage reduction catalyst.
The NOx storage reduction catalyst requires that its temperature be sometimes raised in order to recover from SOx poisoning. When the temperature of the NOx storage reduction catalyst is to be raised, the air-fuel ratio is adjusted; for example, the air-fuel ratio in the three cylinders of the first cylinder group is made rich (lower than the stoichiometric ratio) and the air-fuel ratio in the three cylinders of the second cylinder group is made lean, in such a manner that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes equal to the stoichiometric air-fuel ratio. At this time, the degree of richness and the degree of leanness are feedback-controlled via the air-fuel ratio sensors disposed in the first exhaust passageway and the second exhaust passageway so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes precisely equal to the stoichiometric air-fuel ratio.
If the air-fuel ratio of the three cylinders of the first cylinder group is made rich and the air-fuel ratio of the three cylinders of the second cylinder group is made lean so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes equal to the stoichiometric air-fuel ratio as just mentioned, the exhaust gas from the first cylinder group that contains a large amount of unburned HC and CO and the exhaust gas from the second cylinder group that contains a large amount of excess oxygen merge with each other at the NOx storage reduction catalyst. As a result, the large amount of unburned HC and CO is oxidized by the large amount of oxygen, and the thus-generated oxidative reaction heat raises the temperature of the NOx storage reduction catalyst.
In this case, in order to increase the amount of temperature rise of the NOx storage reduction catalyst, it is necessary to increase the degree of richness in the first cylinder group and increase the degree of leanness in the second cylinder group. However, in reality, increasing the degree of richness and the degree of leanness in this manner usually results in deviation from the range of air-fuel ratio that can be accurately detected by the air-fuel ratio sensors. As a result, there arises a problem of failing to control the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst precisely to the stoichiometric air-fuel ratio.
Besides, the NOx storage reduction catalyst also has the function of a three-way catalyst. Therefore, this NOx storage reduction catalyst has functions of simultaneously removing or lessening the unburned HC and CO and the NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is stoichiometric. After the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas have flown into the NOx storage reduction catalyst, these exhaust gases are not quite mixed but remain separate as a rich air-fuel ratio portion and a lean air-fuel ratio portion while in an upstream portion of the NOx storage reduction catalyst. It is not until they reach a downstream portion of the NOx storage reduction catalyst that the air-fuel ratio thereof becomes equal to the stoichiometric air-fuel ratio. Therefore, the unburned HC and CO and the NOx in exhaust gas are removed only in a downstream portion of the NOx storage reduction catalyst.
If the degree of richness of the first cylinder group is increased and the degree of leanness of the second cylinder group is increased, the region where the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are not sufficiently mixed expands to the downstream side of the NOx storage reduction catalyst. In consequence, there arises a problem of having to increase the capacity of the NOx storage reduction catalyst so as to secure a sufficient region where the exhaust gas air-fuel ratio becomes stoichiometric for the purpose of removal of the unburned HC and CO and the NOx.